Basically you just use the cardbacks to make them indistinguishable from a normal card then put a note in if using sleeves (that way they are reusable as other cards) or write on them if not (but then they are obviously not reusable as other cards). You can find descriptions for all the cards released on arkhamdb.com and you can set the language to German. I have done so myself to be able to include a few cards from circle undone which came at a time when I did not have time to play the game and I have been waiting for the re-release of.
Obviously not as nice as having the real cards and unless you are using sleeves you can’t do it for every card, but a cheap way to use that one card you really want to have in your deck without buying a whole expansion for it.
This is the same for me. Feel like a collection of half-baked attempts at mechanics from too many other games. The five turn limit is to short to do anything interesting with them, and makes it so the winner of the game is basically decided after turn 1. I like the theme and there are lots of games that are technically worse, but it just feels kinda mediocre and dull.